Insulation, Radiant and Vapor Barriers - Insulating Cathedral Ceiling

The log house I'm building has a cathedral ceiling with 2x10 rafters 16" oc. The plans call for R30 insulation with baffles to allow air flow.

Is it necessary to use a baffle in every rafter cavity? Or to run the baffle the entire length of the cavity? That could get expensive.

Hi md, This reply involves some opinion as I live in a cold area and have had to deal with many cathedral ceilings. R-30 is conservative, even for WV and this is an area of the house where you can't easily go back and add more, like an open attic. Also, there will be or should be a constant flow of air over this insulation, so, especially for fiberglass, it's insulation properties will be reduced if you do not provide an air baffle the full length. Air moving through fiberglass or any insulation for that matter just kills it's effectiveness.

And yes, you should install a baffle in every bay. The baffles can be home made, as long as they don't create a double vapor barrier. The link below will discuss vapor barriers and retarders.
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders (http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810)
A 2x10 cavity gives you about 8" for insulation with the rest (1 1/4" to 1 1/2") needed for venting. Generally 1 inch is considered a minimum and with the length of tour rafters I would not go that low. You didn't say what you are using for insulation, but at r=3.5 per inch you get only 28 and remember this is the ceiling so the heat loss is higher due to the higher temps. 68 degrees below could result in 80 degrees up top. I have often seen an additional layer of rigid insulation used to cover the bottom of the rafters and insulation. It eliminates some of the thermal bridging through the wood and adds a nice extra bump to your r-value.

Hey, enjoy your new home and don't hesitate to go back to the log folks to get them to fill in the details, it's their log home as well.

Bud

Thanks for the comprehensive reply. I had pretty much come to the same conclusions as your advice.

How would I make homemade baffles?

Lost that post so will try again. You can install a ledge on each side of each bay with a strip of 1 1/2" wood say 1/2" thick, then staple of mail something to it to keep the insylation from blocking the air flow. Fanfold insulation, 1/2", or 1" rigid, or even cardboard, as lond as it is not a vapor barrier. Thin rigid insulation will allow some moisture to pass through it as the previous link suggested.

Here is another helpful link:
http://www.southface.org/web/resources&services/publications/factsheets/25_insulateceilings_4pdf.pdf

Bud

Great, thanks for the info.